Ben Miles as Thomas Cromwell and Lydia Leonard as Anne Boleyn in the RSC production of Wolf Hall. Photograph: Alastair Muir/Rex

Stage versions of Wolf Hall and Bring Up the Bodies will transfer to London's West End in May, it will be announced on Thursday, but fans of Hilary Mantel's all-conquering Thomas Cromwell novels will have to wait a while longer than they might have hoped to read the third instalment of the trilogy.

"I have to ask my readers to be patient because the third book is highly complex," the writer said as she spoke of the stage transfer. "I have to let it evolve. I don't want to force it ." Mantel said she hoped to have the final part of the trilogy "substantially finished within 18 months", but added: "There's nothing about the process that's predictable."

Mantel has spent much of her recent time in Stratford where the RSC is staging productions of her first two books to full houses. But she said it was time well spent.

"It is an enormous privilege. The last six months have been the best six months of my writing life. I've learned so much and it has been really exhilarating as well as the buzz of being in the audience many nights getting the kind of feedback an author of a novel never gets.

"There's something really inspiring for me in the whole process. It has really powered me up as a writer."

The RSC said it would transfer the plays to the Aldwych theatre, for 20 years the company's London home, on 1 May. They replace Andrew Lloyd Webber's musical Stephen Ward, which is closing early on 29 March.

The plays are directed by Jeremy Herrin and have been adapted from Mantel's two Man-Booker-winning books by Mike Poulton. Much of the current RSC company is to also transfer including Ben Miles as Cromwell and Nathaniel Parker as Henry VIII.

Mantel said her experiences would make for a better finale to the trilogy. "I've had so much inspiration from what happens on stage night by night – and I've understood things that I don't think I did understand before about the characters.

"It is true I've invested a lot of time in the production but that investment has really paid off. When I'm here I write all the time. I'm just unspooling the third book into my notebook. I do need to go through a phase where I go back and lock myself in a room, sit at a desk, pull it all together and organise it and over the next few months I will go through that process. But while this experience is here I should really take advantage of it and learn as much as I can, absorb as much as I can."

Mantel published Wolf Hall in 2009 and Bring Up the Bodies in 2012 and they have sold 1.8m copies in the UK alone.

Mantel said success did mean "every time you open your mouth people are waiting to jump on you but I can live with that – I'm not going to be intimidated in that way".

The plays have also proved a success. On Saturday David Cameron was in the audience, and there was applause for the character who said the line: "Governments should always listen to the voice of the people."

RSC members' booking for the London run (between 1 May and 6 September) opens on Thursday and public booking on Monday 10 March.